The process in which metals are eaten up gradually by the action of air, moisture or a chemical such as acids on their surface is called corrosion.
For example, rusting of iron metal is the most common form of corrosion.
4Fe + 3CO2 + 2x H2O → 2Fe2CO3. xH2O
The rusting of iron is a redox reaction.
The black coating on silver and the green coating on copper are other examples of corrosion.
Effects of corrosion: Damage to bridges, iron railings, bodies of vehicles, ships and all objects made of metals like iron.
Why do we apply paint on iron articles?
If iron particles are left exposed, their surface comes in contact with the atmospheric oxygen and in the presence of moisture it forms Iron(III) oxide or rust. They are therefore painted to prevent them from rusting as the paint does not allow the surface to come in contact with oxygen and moisture and thus prevents rusting.
The slow oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
Prevention of Rancidity
- By adding anti-oxidants to foods containing fats and oils: Usually substances that prevent oxidation (antioxidants) are added to foods containing fats and oil. The two common anti-oxidants are BFIA (Butylated Hydroxy-Anisole) and BHT (Butylated Hydroxy-Toluene).
- Keeping food in air tight containers helps to slow down oxidation.
- By packaging fat and oil-containing foods in nitrogen gas: Chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidized.
- By keeping food in a refrigerator.
- By storing foods away from light.
Oil and fat-containing food items are flushed with nitrogen. Why?
Fried food items containing oil and fat get oxidized and become rancid in the presence of air or oxygen. As nitrogen is a comparatively unreactive gas as compared to oxygen, it prevents the oxidation of food items and hence prevents food from becoming rancid. That is why oil and fat containing food items are flushed with nitrogen gas.